Epoxy resins are well known for use in making advanced or high performance composites comprising high strength fiber made of glass, boron, carbon or the like. Structures made of these composites can weigh considerably less than their metal counterparts at equivalent strength and stiffness. Higher modulus epoxy composites, however, have been relatively brittle. This brittleness restricts their wider application because, for example, damage tolerance, an important property of flight critical components in aircraft, is related to brittleness of the component.
One approach in making epoxy composites tougher has been to introduce functionally terminated rubbery polymers into the epoxy resin formulations. The thermosets resulting from these formulations, while having increased toughness, have reduced modulus.
Another approach has been to incorporate engineering thermoplastics into the epoxy resin formulation. Various thermoplastics have been suggested and the use of a polyethersulfone as the thermoplastic modifier for epoxy resin formulations was studied by C. B. Bucknall et al and is discussed in the British Polymer Journal, Vol. 15, March 1983 at pages 71 to 75. Bucknall et al's studies were carried out on cured epoxy-polyethersulfone blends prepared from trifunctional and/or tetrafunctional aromatic epoxides, diaminodiphenylsulfone or dicyandiamide hardener and various amounts of Victrex 100P manufactured by ICI Ltd., said to be a relatively low molecular weight grade of polyethersulfone. The studies showed that phase separation occurred in certain of the cured epoxy-polyethersulfone blends and that some of the cured blends exhibited distinct nodular morphological features. Analyses indicated that the polyethersulfone was concentrated in the nodules and Bucknall et al surmised that the nodules were not formed by polyethersulfone alone but by a crosslinked epoxypolyethersulfone copolymer. Bucknall et al found no clear correlation between composition and mechanical properties such as elastic modulus, fracture toughness and creep of the cured blends and concluded that the addition of the polyethersulfone had little effect on the fracture toughness of the resin mixtures, irrespective of the degree of phase separation or the morphology.
Yet another approach for improving the mechanical properties of cured epoxy resins is described in U.S. Pat. No. 4,330,659 to King et al. King et al disclose using as the hardener for epoxy resins the reaction product of diaminodiphenylsulfone with diglycidyl ethers of polyhydric phenols. The cured resins prepared from mixtures of a "modified" hardener obtained by adducting the coreaction product of the diglycidyl ether of bisphenol A and additional bisphenol A with diaminodiphenylsulfone, and tetraglycidylated methylene dianiline are reported to have increased impact strength and toughness as compared with the cured resins obtained using unmodified diaminodiphenylsulfone as the hardener.